The thiocyanate ion shows a specific color in the presence of a certain kind of metal, for example, a metal of the iron group. When a slightly yellow aqueous solution of ferric nitrate or a light carmine aqueous solution of cobalt chloride is added to a colorless aqueous solution of potassium thiocyanate, for example, an orange brown or red purple aqueous solution attended with no precipitate is formed, respectively. Because of these characteristic color reactions, the thiocyanate ion and these coloring metal ions have been utilized for simple qualitative analyses of the individual ions. From the past, aqueous colored colloidal dispersions and aqueous solutions of certain kinds of organic dyes were used as ink compositions. Whereas, an ink composition containing thiocyanate ion has not yet been known heretofore.
The present inventor previously provided an ink composition containing cobalt thiocyanate as a predominant ingredient (Japanese Patent Publn. No. 49-27527; Japanese Pat. No. 765,826). This ink composition has a character that letters and lines drawn with the ink composition can easily be erased, if necessary, by wiping with cloth, paper or fibers impregnated with water.
In this ink composition, however, there is such a disadvantage in actual use that if the amount of cobalt thiocyanate used is saved and a solution of a lower concentration is used as ink, the written letters are not colored in blue immediately but are colored for the first time when the solvent is evaporated to increase the concentration. To overcome such disadvantage, the use of an ink wherein the concentration is increased to at least 30% is necessary. In such case, however, fluidity of the ink becomes poor because of such high concentration so that difficulty arises in charging a pen core with the ink. When the concentration of cobalt thiocyanate becomes higher by gradual evaporation of the solvent on the repeated use of a pen, the ink will not come out clearly so that the written letters become thin. Considering these situations, inks containing cobalt thiocyanate as a predominant ingredient is used usually in a concentration of about 20%. In this case, however, difficulties arise in actual use as described above because of such too low concentration.